Critical machinery increasingly requires that the lubrication system is monitored real-time for indicators of mechanical as well as fluid degradation. Because external factors have an extensive influence on the lubricant during operation, knowing the particle size distribution and material properties of the contamination plus other properties related to lubricant quality such as water saturation, enables the user to evaluate the precise condition of the system. For example, the mechanical components of a helicopter gearbox operate under extreme load, speed and environmental conditions causing rotating component wear that can progress very rapidly to result in a sudden, catastrophic failure. Corrosion precursors, especially water, significantly affect material strength properties in gearbox components, thereby reducing the component's load carrying capacity and ultimately shortening its useful lifetime. Bearing failures often cause reliability issues and can cause catastrophic failures of the entire system. Real time sensors can provide the capability to continuously monitor the lubricant flow to detect the onset of a premature failure and provide for safe shut down. On-line sensors can trend wear debris size and the rate of production as well as identify corrosion precursors. This capability will improve maintenance action and component replacement recommendation, thus increasing asset readiness and reducing total ownership costs.
Engine and gearbox wear is commonly monitored using magnetic chip detectors placed directly in the oil flow that can detect metallic particles approximately 200 μm in size and greater. Inductive sensors are available for detection of both ferrous and non-ferrous metals as well. However, these methods cannot detect non-metallic wear debris particles. This poses a limitation since new high performance engines have begun to utilize new materials and ceramic bearings that offer considerable weight savings, and thus increased fuel efficiency. Obviously, legacy health monitoring systems using magnetic or inductive sensors are not capable of detecting the failures of these non-metallic materials.
Hence there is a need to monitor the health of these non-metallic components. Also, it is desirable to be able to monitor all of the circulating fluid (oil), not just a portion of it, to ensure that all the larger more serious wear debris particles are detected. As a general rule, the larger the particle size, the more serious the potential failure condition, but correspondingly occur more rarely. A sampling strategy is not appropriate to address the rarely occurring larger particles. Thus, all of the oil flow must be monitored; an in-line system for monitoring is the best option. In addition, there is also a need to monitor the lubricant quality such as water saturation which accelerates the aging process of metallic engine components.
Prior art technology does not address all the above requirements. A number of patents and papers relate to acoustic monitoring of fluid flow. A method of this type is disclosed in British Patent 1,012,010 (1963) which describes a method and equipment for counting and measuring particles in various measurement zones along the acoustic axis of an ultrasonic transducer in the suspension. By using suitable time windows when receiving the reflected acoustic signals, the particles in a predetermined number of measurement zones are counted. By making use of a different threshold voltage for each time window, a minimum size for the particles to be counted is selected for each zone. Assuming that the particle distribution is the same in each zone, and only one particle is within the measurement zone, a rough estimate of the number of particles, subdivided according to particle size is obtained.
Other systems characterize the type and shape of particles in the suspension by evaluating the angle-dependent reflection behavior of the particle. U.S. Pat. No. 4,381,674 (1983) and U.S. Pat. No. 4,527,420 (1985) describe a bistatic arrangement for target material identification on the basis of the ratio of the outputs of two transducers. U.S. Pat. No. 4,339,944 (1982) covers particle discrimination on the basis of comparing spectral characteristics of the reflected pulse with previously acquired spectra of known particles. This is also described in Nemarich, C. P., J. C. Tuner, and Whitesel, H. K., “Evaluation of an On-Line Ultrasonic Particle Sensor Using Bearing Test Data”, 41st Meeting of the Mechanical Failures Prevention Group, Patuxent River, Md. (1986). In U.S. Pat. No. 6,205,848 (2001) a large measurement Volume is described such that the angle of incidence varies as a function of the lateral position. If a particle in the flowing suspension is exposed various times in succession by an acoustic signal, the successive reflection signals differ as a consequence of the angle-dependent reflection.
Prior work using ultrasonic transducers for wear debris measurements performed by Innovative Dynamics Inc. [(1) Edmonds, J., M. Resner, and K. Skharlet, “Detection of precursor wear debris in lubrication systems”, IEEE, 2000; (2) Edmonds, J., J. Gerardi, G. Hickman, “Helicopter/Tiltrotor Gearbox Debris Monitoring”, Navy SBIR Phase I IDI Final Report, 1995) has shown the ability to measure particles down to 5 um in diameter, and when combined with inductive sensors provides full spectrum wear debris monitoring capability, allowing one skilled in the art to be able to identify and differentiate both metallic and non metallic wear debris particles.
These methods all use focused ultrasonic transducers to estimate the particle concentration and the particle size distribution based on statistical sampling of the flow. These methods are limited by the shape of the acoustic beam and only a partial volume of the fluid that passes by the transducer is monitored. Particles outside the focus region, including larger size particles indicative of impending failure, can therefore not be detected. Also, while some of these methods can differentiate air bubbles and solid particles because they have distinct spectral shapes, these methods do not currently sample fast enough to detect all debris if the flow rate or debris concentration is relatively high, thus requiring complex high speed sampling hardware. Although an ultrasonic transducer responds to all solid debris and current designs are unable to reliably differentiate between metallic debris and non-metallic debris, air bubbles, or water.